This invention relates to the treatment of sour hydrocarbon distillate to remove mercaptan compounds by contacting the distillate with an alkaline solution.
More particularly, this invention relates to the removal of such mercaptans by a two-stage oxidation contact wherein the first oxidation of the mercaptan compounds occurs while the hydrocarbon and the alkaline solution are in contact with a bundle of a plurality of elongated fibers.
The art relating to the treatment of sour distillate hydrocarbons is well developed and the processes therefor are the subject matter of many patents. For example, U.S. Pat. Nos. 3,758,404, 3,977,829 and 3,992,156 describe mass transfer apparatus and processes involving the use of fiber bundles which are particularly suited for such processes. It is an improvement over the inventions described in such patents and others that this invention is made for the treatment of hydrocarbon distillates contaminated with mercaptan compounds requiring more treatment residence time than are practically possible in a single phase bundle treating process as described in the above mentioned patents in order to accomplish sufficient removal. While satisfactory treatment could be accomplished in a series of such treatment apparatus. it then becomes very expensive and uneconomic. Thus the instant invention is an improvement over such described methods.
The treatment process of this invention will be recognized by those skilled in the art to be particularly suitable for the treatment to effect the catalytic oxidation of the offensive mercaptans contained in sour hydrocarbon distillate such as for example gasoline, including natural straight run and cracked gasolines, naphtha, kerosene, jet fuels, fuel oil and the like.
Commonly used processes for treating sour hydrocarbon distillates entail treating the distillate in contact with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour distillate and the catalyst-containing aqueous alkali metal hydroxide solution provide a liquid-liquid system wherein mercaptans are converted to disulfides in the presence of an oxidizing agent --usually an oxygen contained gas dissolved in the hydrocarbon being treated. Sour hydrocarbon distillates containing more difficulty oxidizable mercaptans are more effectively treated in contact with a metal phthalocyanine catalyst deposited on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The distillate is treated in contact with the supported metal phthalocyanine catalyst at oxidation conditions in the presence of an aqueous alkaline solution. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is more often air admixed with the distillate to be treated, and the aqueous alkaline agent is most often an aqueous alkali metal hydroxide, or caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in a caustic-wetted state.
The prior art recognizes that there are limitations on the ability to treat a sour hydrocarbon distillate with a catalytic composite consisting of a metal phthalocyanine disposed on a support material such as the relatively short catalyst life plugging of the catalyst bed, and the required utilization of aqueous-phase alkaline reagents. Various improvements have been developed to further enhance the sweetening ability, including the use of certain additives in the distillate treating process, for instance, a method comprising contacting the distillate at oxidation conditions with a supported metal chelate mercaptan oxidation catalyst and anhydrous ammonia in the absence of an aqueous phase. (U.S. Pat. No. 4,502,949).
It has now been surprisingly discovered that the difficulty oxidizable mercaptan compounds are economically and effectively oxidized and removed to meet mercaptan content specifications of distillate hydrocarbons by the invention as described hereinafter.